1. Field of the Invention
This invention relates to a power generation system using a fuel cell. More particularly, the invention relates to a fuel cell power generation system including an auxiliary fuel cell stack for generating sufficient electrical power to start fuel cell subsystems such as an air compressor and to operate loads that need power during shut down conditions.
2. Description of the Related Art
Fuel cells are energy conversion devices which produce heat and direct current electrically from a chemical fuel and an oxidizer through a continuous electrochemical reaction. There are multiple types of fuel cells and a typical fuel cell stack is made of a number of cells wherein each cell has an anode, a cathode, and an electrolytic layer therebetween. The main difference between various fuel cell stacks is the type of electrolytic layer used. In Proton Exchange Membrane (PEM) fuel cells, fuel containing hydrogen is supplied to the fuel chamber at the anode and an oxidant gas containing oxygen is supplied to the air chamber at the cathode to generate electric power.
A fuel cell power generation system includes a fuel cell and several auxiliary units (balance of plant) to provide logistics and support for safe and reliable operation. These subsystems include: fuel handling, oxidant handling, water management, thermal management, and a control system. The fuel handling subsystem consists of a fuel storage tank, valves, and regulators to control pressure and fuel flow. The oxidant handling subsystem supplies air to the cell by means of a compressor or air pump. The water management subsystem consists of a condenser, tank, and a pump needed for collecting water exiting the fuel cell stack. The thermal management subsystem is required to cool the stack and typically consists of a heat exchanger, pump, and coolant. The fuel cell system, inclusive of the support peripherals, also requires a control system, typically a controller.
It is generally known that a primary difficulty associated with fuel cell power generation systems is the initiation time of the electrochemical reaction within the fuel cell stack. A fuel cell power generation system which includes the above mentioned subsystems requires a substantial amount of electrical power to operate auxiliary systems, with the most demanding being the oxidant handling and thermal management subsystems. Electrical loads of these units can be over 20% of the total fuel cell output power. Further, power must first be delivered to the support subsystems to start up the fuel cell stack. Current PEM fuel cell systems require a large battery pack to support these start up loads, as well as the shutdown condition (i.e., key-off) loads.
It has been proposed to use a 12 volt battery subsystem to power electrical components in a fuel cell power generation system. However, to increase overall system efficiency and to assure initiation, the above mentioned subsystems of a fuel cell system are operated at a high voltage. Therefore, it is not uncommon for fuel cell battery storage systems and the above mentioned subsystems to be operated in the 300 volt range. However, 12 volt systems are incapable of directly providing the requisite power output to initiate a fuel cell. Further, any DC voltage conversion proves to be inefficient and likely to quickly drain a 12 volt storage battery under difficult starting conditions.
Another difficulty associated with fuel cell power generation systems is that when any fuel cell is to be utilized in an isolated environment, such as in vehicles, the fuel cell may be subject to extreme winter temperatures, such as temperatures from 0xc2x0 C. (+32xc2x0 F.) down to as low as xe2x88x9240xc2x0 C. (xe2x88x9240xc2x0 .F). Typically, a fuel cell may not be stored below about 0xc2x0 C. (+32xc2x0 F.) without freezing. Therefore, when experiencing ambient temperatures below about 0xc2x0 C. (+32xc2x0 F.), the fuel cell may freeze. While it has been proposed to use various anti-freeze solutions to solve this problem, this solution requires additional anti-freeze supply and transport equipment within the fuel cell power generation system.
What is needed, therefore, is an improved system for providing start up power to fuel cell support subsystems and for providing power to support key-off system loads for land based or vehicular fuel cell applications. It is further desirable to provide a method of fuel cell operation in which the problems associated with low temperature use are eliminated without anti-freeze solutions.
The foregoing needs are met by a fuel cell power generation system that provides a novel approach for providing start up power to fuel cell support subsystems and for providing power to support key-off system loads applicable for land based or vehicular fuel cell applications. To meet the demand of the initial power load, an auxiliary fuel cell stack is incorporated into the overall fuel cell power generation system architecture. One exemplary auxiliary fuel cell stack for the fuel cell power generation system is a convective solid polymer fuel cell that operates at ambient temperature and pressure conditions without humidification. The auxiliary fuel cell incorporates an electrolyte membrane different from traditional proton exchange membrane fuel cells in that the auxiliary fuel cell can diffuse oxygen without forced airflow. Only pressurized fuel is required to operate the auxiliary stack.
To start up the fuel cell system, power generated by the auxiliary fuel cell stack is delivered to the fuel cell support subsystems. The subsystems begin operation and initiate the primary fuel cell stack. Oxidant and fuel delivery to the primary fuel cell stack is ramped until the primary fuel cell stack reaches normal operating conditions. At that time, fuel delivery to the auxiliary fuel cell stack is turned off, shutting down the auxiliary fuel cell stack. The primary fuel cell stack then provides subsystem power. Another function of the auxiliary fuel cell stack is to provide the power for the land based/vehicle key-off loads. These are electrical systems which require power even under normal/emergency shut down conditions (integrated computers, engine control module, clocks, theft alarm, etc.) and that require power when the vehicle or land based power generation system is off. In this shutdown mode, power generated by the auxiliary stack is used to support these loads. One highly advantageous use of the power generated by the auxiliary stack during shutdown conditions is the running of a coolant fluid heater and a coolant pump to circulate heated fluid throughout the primary fuel cell stack to avoid freezing of the primary fuel cell stack.
A fuel cell power generation system in accordance with the invention includes a primary fuel cell stack for generating a first quantity of electric power, an auxiliary fuel cell stack for generating a second quantity of electric power, a fuel handling subsystem for feeding a fuel containing hydrogen to the primary fuel cell stack and the auxiliary fuel cell stack, an oxidant handling subsystem for feeding an oxidant containing oxygen to the primary fuel cell stack, and a controller electrically connected to the primary fuel cell stack, the auxiliary fuel cell stack, the fuel handling subsystem and the oxidant handling subsystem for controlling operation of the fuel cell power generation system.
The primary fuel cell stack includes an anode, a cathode, an electrolytic layer positioned between the anode and the cathode, a fuel flow path adjacent a side of the anode opposite the electrolytic layer, and an oxidant flow path adjacent a side of the cathode opposite the electrolytic layer. The auxiliary fuel cell stack includes a fuel electrode, an oxygen electrode, an electrolytic member positioned between the fuel electrode and the oxygen electrode, a fuel distribution path adjacent a side of the fuel electrode opposite the electrolytic member, and an oxidant distribution path adjacent a side of the oxygen electrode opposite the electrolytic member. The fuel handling subsystem is configured to feed a fuel containing hydrogen into the fuel flow path to bring the fuel into contact with the anode of the primary fuel cell stack and to feed the fuel into the fuel distribution path to bring the fuel into contact with the fuel electrode of the auxiliary fuel cell stack, whereby the contacting of the fuel with the fuel electrode generates the second quantity of electric power from the auxiliary fuel cell stack. The oxidant handling subsystem includes a compressor for feeding an oxidant containing oxygen into the oxidant flow path to bring the oxidant into contact with the cathode, whereby the contacting of the fuel with the anode and the contacting of the oxidant with the cathode generates the first quantity of electric power from the primary fuel cell stack.
The controller executes a stored program to sense a startup signal for the fuel cell power generation system, to provide a fuel delivery signal to the fuel handling subsystem to initiate feeding of the fuel into the fuel flow path of the primary fuel cell stack and into the fuel distribution path of the auxiliary fuel cell stack in response to the sensed startup signal, and to apply at least a portion of the second quantity of electric power generated by the auxiliary fuel cell stack to the compressor to run the compressor to feed the oxidant into the oxidant flow path of the primary fuel cell stack in response to the sensed startup signal. In another embodiment of the invention, the controller may be electrically connected to a load and the auxiliary fuel cell stack, and the controller can execute a stored program to sense a load demand from the load (which may be a key-off load), to provide a fuel delivery signal to the fuel handling subsystem to initiate feeding of the fuel into the fuel distribution path of the auxiliary fuel cell stack in response to the sensed load demand, and to apply at least a portion of the second quantity of electric power generated by the auxiliary fuel cell stack to the load in response to the sensed load demand.